In the process of interfolding and stacking relatively thick or air-impermeable one-ply or multi-ply paper sheets on an interfolding machine, the paper sheets to be interfolded are not always securely bonded together but may displace relative to one another. As a result, wrinkles, folds, and looseness frequently occur between two interfolded paper sheets or between the plies thereof, giving the interfolded paper sheets an unsmooth appearance.
A conventional way of solving the above problem is to perforate the paper sheets and form air vents thereon in order to increase the air permeability thereof, so that an enhanced suction is produced between two paper sheets or between the plies of the paper sheets, making them bonded together. For example, U.S. Pat. No. 6,213,927B1 discloses an interfolding method of sheet material not or not enough permeable to air and machine used to carry out such method. The interfolding machine has two folding rollers, which are provided on respective outer peripheral surfaces with a plurality of sucking spots; and two rollers separately located above the two folding rollers, and having a plurality of needles provided on the outer peripheral surfaces thereof corresponding to the sucking spots on the folding rollers, so as to perforate the sheet material not or not enough permeable to air at predetermined positions.
However, the perforations or air vents are useful only when they are formed within a particular narrow area on each paper sheet at where the paper sheet is folded. Moreover, the forming of perforations on the paper sheets tends to cause breaking and accordingly, poor quality of the paper sheets.